Every 100 years is called a century, made up of 10 decades.
bidecadal
True.
The phrase "once every 3 years" can be expressed as "triennial." This term refers to an event or occurrence that happens every three years. Alternatively, you could also say "every three years" for clarity.
A false rumor is floating around the Internet that this happens only once every 823 years. The reality is that it happens whenever the first day of October is on Friday, which happens on average once every seven years. The last time for this "rare" event was 6 years ago, back in 2004. And the time before that was 5 years previously, in 1999.
Every 200 years
An event occurring every two years is referred to as a biennial event. This pattern indicates that the event happens once every two years.
A triennial event happens every three years.
Something that happens every six years is sexennial.
It is a triennial event. A triennial event happens three times a year.
bidecadal
nothing
Semicentennial
True.
I think you are asking about a census.
Leap year
Chicago Cubs in the World Series?
the word is novennial....